Efficiency of an internal combustion engine may be controlled by adjusting spark timing and an exhaust dilution that is combusted by the engine. Spark timing may be defined as generating a spark at a specific time relative to a position of a piston in a combustion chamber. A preferred spark timing occurs when the engine has a maximum output power. Output power is directly proportional to pressure within a combustion chamber of the engine. To obtain optimal pressure in the combustion chamber, the spark ignites the air/fuel mixture at a time before the piston is at top dead center.
The engine system may also dilute the air/fuel mixture to improve engine efficiency. Exhaust dilution may improve engine efficiency by reducing pumping losses. When exhaust gases are introduced during an intake stroke of a combustion event, the engine induction vacuum is decreased, thereby reducing pumping power losses of the engine. An engine control system may use an exhaust gas recirculation (EGR) system to re-circulate exhaust gases back into the intake manifold. In addition, a set of cam phasers may be used to control valve timing in order to retain exhaust gases in the combustion chamber.
Optimal spark timing and dilution values of the engine are typically determined experimentally in a single set of air conditions. However, engines do not always operate in the same set conditions due to geographic location, seasonal changes, and/or other circumstances. When engine performance is not optimized, the engine system may experience reduced fuel economy and sluggish engine output.